1. Field of the Invention
This invention relates to a motor control apparatus for controlling the speed of a motor. More particularly, the invention relates to a motor control apparatus capable of eliminating the influence of a disturbance torque.
2. Description of the Prior Art
A control apparatus of a capstan motor or a drum motor used in a VCR must keep the speed of the motor fixed. FIG. 11 is a schematic block diagram of a conventional speed control loop of such a motor. In FIG. 11, element 100 is a capstan motor or a drum motor used in a VCR; element 110 is a frequency generator for generating a frequency signal FG, whose frequency is in proportion to the speed of the motor 100; element 120 is a period detector for detecting the period of the frequency signal FG; element 130 is a comparator for calculating a period error .DELTA.T.sub.FG, which is the difference between an inputted desired period T.sub.r and the detected period T.sub.FG from the period detector 120; element 140 is an arithmetic unit for calculating a control signal c which is, for example, equal to (K.sub.p +K.sub.i /S).multidot..DELTA.T.sub.FG, wherein K.sub.p is a proportional gain, K.sub.i is an integral gain and s is a Laplacian; element 150 is a motor drive circuit for supplying the motor 100 with a drive current I.sub. a according to the control signal c.
In the control loop of FIG. 11, a disturbance torque, which is caused by tape transportation, motor torque ripple, external vibration and so on, is applied to the motor 100. The influence of the disturbance torque appears as a fluctuation in the motor speed. Even though a fluctuation in speed is produced by the disturbance, the overall control loop acts in a direction to reduce the fluctuation in speed. The higher the gain of the control loop, the greater the degree to which the disturbance is suppressed. However, there is a limit to the gain, for the overall system will fall into an oscillatory state if the control loop gain is made too high. In other words, if the disturbance torque is large, the fluctuation in motor speed cannot be suppressed sufficiently because the degree of suppression cannot be raised.
Recently, various motor control apparatus, which are capable of suppressing the influence of the disturbance torque by using a disturbance torque observer, have been proposed. For example, one of such apparatus is described in "A Robust Decentralized Joint Control Based on Interference Estimation," Proc. IEEE Int. Conference, Robotics and Automation, vol. 1, 326-331 (1987). In such apparatus, a disturbance torque observer estimates the disturbance torque applied to a motor according to the speed of the motor and the drive command fed to the motor. The drive command is corrected in accordance with the estimated disturbance torque from the disturbance torque observer. As the result of the correction, the influence of the disturbance torque is cancelled and a fluctuation in the motor speed is eliminated.
However, as shown in FIG. 11, a motor control apparatus used in a VCR keeps the speed of the motor fixed by reducing to zero the difference between the desired period T.sub.r and the detected period T.sub.FG. Therefore, the conversion of the detected period into a speed is necessary for applying the conventional disturbance torque observer to a motor control apparatus used in a VCR. This conversion makes the hardware or an operation sequence of a software program used for the motor control complicated. Furthermore, it is difficult to realize a process of estimating the disturbance torque according to the conventional disturbance torque observer with such a processor as used in a VCR for home-use, whose performance characteristics are not very high, because the constitution of the conventional torque observer is complex.